Invasive species arrive in our communities often without any warning, settle in for long destructive stays, and are hard to dislodge. Think of Dutch elm disease, purple loosestrife, zebra mussels, and now emerald ash borer. Successful management of any introduced pest depends on early detection... forewarned is forearmed!
An initiative to get a jump on this most recent pest is the new ‘WaspWatcher’ program used to spot new infestations of the emerald ash borer beetle before it can establishes itself as a serious pest.
The beetles are virtually undetectable ...at least until it is too late. Larvae feed beneath the bark of our native ash trees and only emerge as adults (high in the tree's canopy) during summer. The larval feeding eventually girdles and kills our ash trees.
A native ground-nesting wasp, Cerceris fumipennis, is providing a handy solution to our beetle detection problem. This wasp will prey on the adult emerald ash borers (as well as related native beetles) and carry them, paralyzed, back to its burrow. The paralyzed beetle is then stored underground as food for the wasp's larva.
You can be a wasp watcher.
Once nests are located in your vicinity you can watch the wasps as they return to their nests with prey. You can capture and send us what beetles are being collected. An equally convenient aspect of this wasp's behaviour is that it won't sting humans, even when handled. So, provided we find the needed nests, Cerceris fumipennis is now a valuable colleague in the search for pest beetles.
There is a broader implication of this wasp watching. Just as volunteers do feeder watches, frog counts, loon surveys, and butterfly monitoring using these as indicator species of a healthy planet so you can become a scout for wasp colonies in your area and, once found, we’ll have an early-warning system not only on the destructive Emerald Ash Borer beetle pests but also an inventory of what other beetles are in the area that the wasps are bringing back to their nest. Should one subsequently be declared a pest, we’ll know where they can be found. Forewarned is forearmed!
What are you watching for?
Cerceris fumipennis is a solitary ground-nesting wasp. Each lone female constructs and attempts to maintain a single subterranean nest for the duration of the flight season. Her solitary nest is in close proximity to others, forming a neighbourhood or informal colony of nests. The nest’s entrance is easily visible, marked by a small circular mound of earth. This hole leads into a vertical, pen-sized burrow that descends for about 3 cm before bending to a 45 degree angle and continuing downwards for a further 9 cm, at which point it levels out and becomes clogged with a loose sand plug. Brood cells with one pupa and paralyzed beetles as food per cell are constructed off this central tunnel.
Cerceris fumipennis is distinguished by five conspicuous characteristics:
- It is large, about the size of common yellowjacket wasps.
- It has dark smoky, blue/black wings (i.e.fumipennis).
- The wasp’s body is predominantly black except for a few yellow markings.
- It has a conspicuous, single broad creamy yellow abdominal band.
- Females have three creamy yellow patches between the eyes; while males are marked with two yellow triangles abutting their eyes.
When are the wasps out?
In Ontario the flight season is a period from late-June to mid-September when adults maintain burrows and provision their larva in subterranean chambers. The larva cocoons are likely watertight in order to survive extreme dry or wet periods over winter. They emerge in late June.
This 3-5 week period (July and early August in Ontario) is when most buprestids are collected and most brood cells are provisioned, although daily success varies. The colony is relatively stable during this period but the fate of individual nests remains uncertain. Some nest entrances collapse, shutting out their owners. Other nests are suddenly abandoned, either because the owner left to establish a new nest elsewhere or perished while foraging. A few new nests do appear during this period but the trend is a continual decrease in the number of active nests. We refer to this decline in the number of active nests as ‘colony decay’.
By mid-August colony decay reduces the number of active nests per site to one quarter of the number present during the first week of July. The remaining nests are often widely spaced but well maintained. The female wasps continue to complete foraging flights, but this period is marked by a change in the duration of foraging flights with a few very long flights and many short trips. In addition, the number of beetles collected per day is significantly reduced.
Early September in Ontario is the final stage in C. fumipennis’ flight season. For the handful of female wasps still alive provisioning has ceased. With ragged wings and sluggish movements the wasps begin to backfill their burrows although neglected burrows quickly collapse on their own. Wind, rain and human activity can seal up an un-maintained burrow in two or three days; yet the remaining wasps seem driven to backfill their nests, possibly to conceal their nest.
Wasp activity increases tenfold once the colony is struck by direct sunlight. Most received almost 10.5 hours of direct summer sun, where the soil surface had to reach at least 29°C before the first few wasps started to fly. C. fumipennis colonies are normally located in open areas with exposure to full sun. The data indicates that most foraging flights (successful or unsuccessful) are completed between 11:00 a.m. and 5:00 p.m. The variables that influence wasp’s activity such as soil temperature and sunlight appear optimal during this period. Efficient biosurveillance work at a C. fumipennis colony would require the human observer to be at the colony between 11 a.m. and 5 p.m., as this would be when wasps are most likely to return with prey from a foraging flight.
An individual wasp completes approximately 4 flights each day (maximum of 14 flights per day) an average of 2 of which result in prey procurement (maximum of 9 successful flights per day) C. fumipennis are more successful at finding prey on some days than others. The wasp’s foraging success may be influenced by the availability of buprestid prey as well as weather conditions. As is the case for many digger wasps, C. fumipennis does not actively forage during rainy weather. In fact most foraging wasps return to their nests moments before large storms arrive. Back in their nests the wasps quickly close the entrance with a soil plug that they push up from within the burrow. This action is similar to the wasp’s nest closure at the end of each day. With the soil plug in place the wasp and its burrow remain relatively dry and undamaged until the rainstorm passes. Possibly stimulated by an increase in soil temperature the wasp removes the soil plug and resumes foraging only after direct sunlight has warmed the soil again.
Warm sunny days do appear to be more productive than cloudy days. (There is an increase in the total number of foraging flights, a high proportion of which produce beetles). But cloud cover does not appear to be the determinant. During the cloudy day the soil surface temperature stayed above 31°C and this appears to have allowed foraging to continue without the influence of direct sunlight. Foraging behaviour responds more to large dips and rises in soil surface temperature rather than simply sun or cloud cover.
How do you gather data?
a) the objective
Visit ‘your’ site a minimum of 3 hot sunny afternoons during the month of July (August if necessary, but July preferred) for beetle collection Collect 50 beetles over those days (15-20 per day – weather-dependent) Put all beetles picked up off the ground on one day in one vial labelled with date and place (and labelled ‘ground’) Put all beetles ‘stolen’ from wasps on one day in a vial labelled with date and place [labelled “stolen”] Put all vials in freezer till end of season, then mail to us (in prepaid boxes)
b) site location
Ground nesting wasps are located in accessible settings, not deep in woods – or poison ivy!
Cerceris fumipennis nests occur in open areas of hard-packed sandy soil surrounded by woody habitat suitable for buprestid beetles. The buprestids being gathered by C. fumipennis are primarily arboreal and it is unlikely that the wasps would nest far from the “grocery store”. Most known colonies are less than 200 m (200 yards) from a forested area.
Use Google Earth or your own knowledge to locate favourable topography in your neighbourhood.
The ground should be a hard-packed with relatively fine, sandy soil (exclude beaches and sand boxes). Sparse herbaceous vegetation is important so areas with a mixture of about 50% bare hard-packed sand and 50% short herbaceous vegetation are best. Focus on areas disturbed by humans: baseball diamonds, informal parking spots, infrequently used roads, sandy roadsides, foot paths and soil around fire pits or open campsites. Elsewhere, colonies occur in a variety of habitats but are most often found on disturbed sites with soils exposed and compacted by human activity such as verges of soil next to asphalt roads or parking lots.
Avoid any freshly dumped mounds of soil or recently landscaped areas. The wasps overwinter approximately 15 cm below the soil surface and seem to build new nest chambers off the hole they emerged from earlier in the summer. For there to be a colony of suitable size the soil below 3 cm must have been left undisturbed for more than a year.
c) finding nests at the site
Finding the first colony will be the hard part but once you have found one colony you will begin to notice them elsewhere. If the wasps are present, you will see them. If they are not conspicuous, then search elsewhere.
Once you have found some colonies you may want to go back and revisit a few of your earlier sites. It is easy to overlook small colonies on days when they are not active such as after a heavy rain. Revisiting possible sites a week later is a good idea.
After finding a promising colony habitat, you will need to locate the nests:
- Walk around any hard-packed, sparsely vegetated soil and look for nest entrances, which are often tucked beside a tuft of grass. Each digger wasp and bee creates their own telltale entrance. Some wasps cover up the openings but C. fumipennis makes a nice little round mound (approximately 4 cm in diameter) much like an ant mound. When you find mounds, check to see if they possess a round central entrance hole, which should travel straight down into the nest, not come in from a side location. The diameter of the hole should fit a golf pencil. A number of digger bees make circular mounds but the entrance holes are much smaller.
- Other insects are helpful indicators when trying to locate a C. fumipennis colony; all are taking advantage of similar soil and light conditions. Keep an eye out for digger wasps buzzing over the ground and excavating nests. Bee wolves, Tachytes wasps, other Cerceris species, digger bees and tiger beetles are found at many C. fumipennis colonies in Ontario.
- Dead buprestids lying around the nests or near the entrances are a good sign that you have found a colony, even if wasps are not active at the site. The female C. fumipennis are encumbered with some of the larger buprestid beetles. If the wasp feels threatened it will drop the large beetle. Without its prey, the urge to “get” a beetle seems to kick in and rather than picking up the dropped beetle, the wasp will head off to catch a new one.
d) confirming nest occupancy
To determine if the nest holes you have found are occupied by our wasp, try two tricks:
- Look down the burrow hole to see if a female is looking out. Often females wait 2 cm below the nest entrance to guard against other females that may want to take their nest. If the wasp looking back at you is a female C. fumipennis she will have a black head with three creamy yellow square patches in a V shape in the middle of her face.
- If the hole is empty place a clear plastic cup over the entrance with a stone on top to prevent the cup from blowing away. Check the cup every five minutes to see if a female is flying around it or buzzing inside it. Do not leave the cups unattended for long periods, as wasps may over-heat and die on sunny days. By catching the female you can easily identify the species.
e) marking nests
If you plan to use the colony to check for Emerald Ash Borer, mark each new nest:
- Use cheap wooden golf tees and coloured collar tabs. Then if the entrance becomes obscured you will know exactly where the nest entrance should be. Writing the nest numbers on the top of the tee or collar will allow you to distinguish each nest.
- Use a GPS to record the geographical co-ordinates.
f) collecting prey
When not excavating a new cell, female C. fumipennis actively forage for prey, traveling to and from the surrounding forest in search of adult buprestid beetles. On the first foray of each day, and sometimes multiple times a day, the departing wasp performs an elaborate orientation flight around the nest entrance. The orientation flight begins with the female wasp slowly poking her head out the nest entrance – much like a groundhog emerging from its burrow. With only her head and upper thorax exposed she looks around, visibly turning her head from side to side. If startled she will instantly retreat down her hole and may not re-emerge for 5 - 10 minutes. If satisfied that the surroundings are safe she then climbs up onto the rim of the tumulus and lifts off vertically. Instead of rapidly flying off to find prey, the wasp flies in ever increasing arcs around the nest, often facing the nest while flying sideways. It is during this period that she re-familiarizes herself with immediate landmarks around her nest and more distant landmarks that will guide her back to the colony.
After completing three or four expanding arcs and reaching a height of approximately 1.4 m above the ground, the female wasp will suddenly dart away from the nest, re-appearing to re-enter her nest minutes or hours later. The wasp will perform a similar orientation flight later in the day particularly if she experienced trouble locating her nest when returning from the previous foray.
Throughout the day C. fumipennis (like many digger wasps) will return from forays without prey, only to travel off on another foray moments later. These seemingly unsuccessful flights may allow the wasp to practice its route back to the nest from various parts of its range. Reorienting itself over the landscape in this manner is particularly important as the wasp forages farther from its nest. Without practicing the route home a foraging wasp is liable to forage over so much new ground that it is unable to find its way back to the nest. While the average foraging range of C. fumipennis is not yet known, many related species are known to travel up to two miles from their nest.
The wasp re-enters the colony flying about a metre above the ground; if carrying prey she may fly sluggishly. Entry into the nest is a simple quick dive, with the wasp sometimes alighting momentarily on the rim of the burrow before dropping in. If threatened during an approach the wasp will drop its prey and fly away. Unless her head is already down the burrow the startled wasp never attempts to flee from danger by re-entering her nest.
Nests can be marked and regulated using a supplied “collar” made using a small 2 x 6 cm plastic or cardboard file card and a standard hole-punch. Holes are made on each end of the tab and it is secured over the nest entrance with a golf tee driven through one hole.
The collar’s hole is large enough to allow wasps without prey to pass through uninterrupted but is small enough to prevent a female returning with prey from squeezing through. In response the wasp (reluctant to release its prey) will buzz and claw at the collar’s opening, alerting a human monitor to the wasp’s return. The bright green EAB adult prey is so distinctive it can be visually identified and then the collar can be moved to one side to allow the wasp to pass into the nest with her prey. Once the female wasp has entered, the collar is repositioned over the entrance and is ready for the wasp’s subsequent exit and next successful return.
Many authors have commented on the great diversity of beetles which C. fumipennis collect. A single colony is capable of collecting multiple species of buprestid each day. During our three years of field work in Ontario and Florida we encountered 9 new prey records for the wasp. The wasp is not hardwired into collecting only species it has co-evolved with but can also adapt to prey on introduced buprestids. Based on prey records C. fumipennis is not restricted to foraging within any one single level of a forest. The wasp will take buprestid species from low shrubs as well as from large canopy trees. As new invasive buprestids arrive in eastern North America they are likely to be added to the C. fumipennis diet. These wasps may also be useful in monitoring for other newly-arrived buprestid pests such as the European Oak Borer.
Since the goal of wasp watchers is to survey native buprestid diversity the observer will need to steal prey from the incoming females without significantly disturbing the wasp’s foraging behaviour. A simple method is to tap the slow flying prey-laden wasp towards the ground with your hand; this will cause the female to release her beetle and then temporarily flee. A somewhat easier method for securing prey without damaging the wasp is to gently catch the incoming female in an aerial net. Once confined to the net the startled wasp will drop her beetle. The ‘robbed’ wasp can be set free and the prey can be picked out of the net; this may be easier than locating a cryptic beetle tossed onto sparsely vegetated ground.
Typically a robbed wasp will return to its nest 13 minutes later, usually without prey, hesitantly approaching before entering the burrow to spend about 13 minutes inspecting the nest before departing on another foray. Selective prey removal can be repeated time and time again without significantly altering the wasp’s provisioning behaviour.